This invention relates generally to safety protection circuits and more particularly, it relates to an improved safety protection circuit for controlling the operation of electrically-operated power tools such as drills, saws and the like.
Power and communication lines disposed in conductive materials, such as metallic conduits and pipes, are employed extensively in modern construction. In the case of a building, these grounded electrically conductive metallic conduit and pipes are frequently installed beneath floors formed of concrete, wood and other similar electrically non-conductive materials when initially erected.
In the subsequent installation of new electrical equipment, it is generally necessary to provide access to the electrical wires in the grounded electrically conductive metallic conduits beneath floors or other structures in order to provide suitable wiring for operation of the newly installed equipment. This entails the necessity of drilling a hole through the floor or other structure and also the electrically conductive conduit for access to the power and/or communication lines. While performing such operations, it is essential not to penetrate or otherwise damage the lines contained in the conduit which can result in short-circuits, serious line damage, explosions, fires, and numerous other hazardous conditions. It is also possible to produce dangerous electrical shocks or even electrocution of a drill operator upon accidential penetration of the power lines.
Until the subject invention a drill operator could only roughly estimate when he would complete drilling through a floor or other structure and the metallic electrically conductive conduit and thus stop the operation of the drill to avoid penetration and damage of the electrical cables contained in the conduit. This is a very unreliable method and creates a potentially hazardous condition each time such a hole is needed. In an attempt to by-pass this dangerous and hit-or-miss technique, a hammer and chisel are frequently utilized for manually cutting a hole in the floor or other structure and then very carefully penetrating the metallic conduit with a second cutting device. While this process is workable, it is inefficient, uneconomical and time-consuming.
It has been known in the prior art to provide a safety circuit for power tools such as drills which will stop the operation thereof upon contact with a conductive material. However, there was no provision made for allowing the drill to be re-activated for penetrating the conductive material. It was thus necessary to by-pass the safety circuit in order to activate the drill for further operation or otherwise a second drill was needed to cut through the conductive material. This was very inconvenient to the operator and caused delays in the number of holes that could be drilled. Accordingly, it would be desirable to provide a safety protection circuit for drills such that the drill can be re-activated after it has stopped due to penetration of the non-conductive material and subsequent contact with the conductive material by simply depressing a push-button switch.
It has, therefore, been found to be desirable to incorporate a safety protection circuit in electrically-operated power tools such as drills and the like which will operate automatically to discontinue the power to the tool and thus interrupt operation of the power tool when it has penetrated a first material of the electrically non-conductive type thus protecting the cables from damage and preventing potentially serious injury to the operator. Subsequently, the tool can be re-activated to penetrate a second material, this time of conductive type. The circuit will be controlled to again automatically shut-off the power to the tool after breaking contact with the second material.
It has also been found to be desirable to provide an indicator system to display the operative condition of the tool, i.e., whether the tool is ready for operation to penetrate a first material or a second material. Further upon the occurrence of various unsafe operating conditions, such as leakage or fault current between the "hot side" wire of the power source and the building ground wire or leakage current between the neutral wire and the building ground wire, it is desired to provide a control means to automatically interrupt operation of the power tool.